Time counter

Time counter

im doing an online expieriment, where u click on a button to start, you pick a color and puts in your information, and sends the info.   in this , i need a timer, that starts when you press the button i made, and starts counting, that count is then able to get send with the form. 

in other words, i need a way to count how long it takes for a person to press start and then send the form, and ofcourse i need the time to be send with the form to the server

anything helps!